librsync

The librsync library does network delta-compression of
streams and files. The algorithm is similar to that used in the rsync and xdelta programs. Unlike
diff and xdelta, librsync does not require access to both of
the files on the same machine, but rather only a short
signature of the old file and the complete contents of the new
file.

librsync ships with a tool called rdiff which does delta
compression from the command line or from scripts.

librsync does not speak the application-specific protocol
used by rsync. They only share an algorithm, not any code.

The current release is
librsync-0.9.6,
which was released in 2004 and seems stable.